The subject matter shown and described in the present application relates to assemblies in which semiconductor chips are packaged and to methods and components useful in making such assemblies.
Modern electronic devices utilize semiconductor chips, commonly referred to as “integrated circuits” which incorporate numerous electronic elements, such as transistors or other active circuit elements. These chips are mounted on substrates which physically support the chips and electrically interconnect each chip with other elements of the circuit. For example, the chip may be mounted in a face-down arrangement, so that a front surface of the chip having contacts thereon confronts a top surface of the substrate and a rear surface of the chip faces upwardly, away from the top surface of the substrate.
The substrate may be a part of a discrete chip package or microelectronic assembly used to hold a single chip and equipped with terminals for interconnection to external circuit elements. Such substrates may be secured to an external circuit board or chassis. Alternatively, in a so-called “hybrid circuit” one or more chips are mounted directly to a substrate forming a circuit panel arranged to interconnect the chips and the other circuit elements mounted to the substrate. In either case, the chip must be securely held on the substrate and must be provided with reliable electrical interconnection to the substrate.
In a microelectronic assembly, structures electrically interconnecting a chip to a substrate ordinarily are subject to substantial strain caused by thermal excursions or cycling between low and high temperatures as temperatures within the device change, such as may occur during fabrication, operation or testing of the device. For example, during operation, the electrical power dissipated within the chip tends to heat the chip and substrate, so that the temperatures of the chip and substrate rise each time the device is turned on and fall each time the device is turned off. As the chip and the substrate ordinarily are formed from different materials having different coefficients of thermal expansion, the chip and substrate ordinarily expand and contract by different amounts. This may cause electrical contacts on the chip to move relative to electrical contacts, such as pads, on the substrate and to terminals on a rear surface of the substrate that connect the substrate to another element, such as another microelectronic element, as the temperature of the chip and the substrate changes. This relative movement can deform electrical interconnections between the chip and substrate, and the another microelectronic element and substrate, and place them under mechanical stress. These stresses are applied repeatedly with repeated operation of the device, and can cause breakage of the electrical interconnections, which in turn reduces reliability performance of the device. Thermal cycling stresses may occur even where the chip and substrate are formed from like materials having similar coefficients of thermal expansion, because the temperature of the chip may increase more rapidly than the temperature of the substrate when power is first applied to the chip.
Improvements can be made to structures that provide for electrical interconnection of a chip to a substrate of a microelectronic assembly and the processes used to fabricate such structures.